Reuse optimized search for scoped results

[justrach]

Summary

• route scoped content search through the optimized searchContent tiers
• preserve enclosing-symbol annotations for scoped MCP results
• add regression coverage for skip-trigram files under max_results pressure

Verification

• zig build test
• zig build -Doptimize=ReleaseFast
• python3 scripts/e2e_mcp_test.py --binary zig-out/bin/codedb --project /Users/blackfloofie/codedb-code-search-sqlite

[github-actions]

Benchmark Regression Report

Thresholds: 10.00% and 50,000 ns absolute delta

NOISE means the percentage threshold was exceeded, but the absolute delta was too small to fail CI.

Tool	Base (ns)	Head (ns)	Delta	Abs Delta (ns)	Status
codedb_bundle	579038	571756	-1.26%	-7282	OK
codedb_changes	62328	63861	+2.46%	+1533	OK
codedb_deps	12499	10014	-19.88%	-2485	OK
codedb_edit	5860	6245	+6.57%	+385	OK
codedb_find	67893	71513	+5.33%	+3620	OK
codedb_hot	111007	109127	-1.69%	-1880	OK
codedb_outline	327961	317272	-3.26%	-10689	OK
codedb_read	100760	95733	-4.99%	-5027	OK
codedb_search	183670	181803	-1.02%	-1867	OK
codedb_snapshot	293991	270010	-8.16%	-23981	OK
codedb_status	106392	109077	+2.52%	+2685	OK
codedb_symbol	63974	62421	-2.43%	-1553	OK
codedb_tree	75606	76653	+1.38%	+1047	OK
codedb_word	79756	76866	-3.62%	-2890	OK